1. Field of the Invention
This invention relates to a transformer for being mounted on a printed circuit board (hereinafter, it is referred as “PCB transformer”.), and particularly to a multi-channel insulated power PCB transformer having plural output channels.
2. Description of the Related Art
When an electronic device comprises a plurality of control circuits, each control circuit is individually supplied with its electric power from the power circuit. Such power circuit includes a multi-channel insulated power transformer which can produce a plurality of independent power from a single power source. The multi-channel insulated power transformer has, typically, coil structures comprising input (primary) coils for being supplied with the electric power from the outside of the device and output (secondary) coils, which is independent each other, for being connected to the control circuit. For the purpose of downsizing electric devices, there has been a demand to make a smaller insulated power transformer having plural output channels for being mounted on a circuit board, i.e. PCB transformer. In such a small power transformer, as compared with a larger power transformer, it is more important to supply highly accurate output power to each output channel to stabilize a drive efficiency.
For example, Japanese Utility Model Kokai No. 04-94713 discloses coil structures in a PCB transformer. An input coil comprises a first half coil and a second half coil, and a plurality of output coils corresponding to each channel are inserted between these input coils that face each other in a radial direction. With respect to the coil structures, the reference mentions that a magnetic flux formed by two input coils can be efficiently coupled to output coils so as to provide higher drive efficiency to the PCB transformer.
For example, Japanese Patent Kokai No. 2000-299233 also discloses coil structures in a PCB transformer. An input coil for one input line is divided into two input coils that face each other in a radial direction and plurality of output coils corresponding to each output channel are inserted between these two input coils. One of the windings in each output coil is mutually disposed on a core along its long axis. The output winding of each output coil is wound by non-inductive winding, such as bifilar-winding or trifilar-winding. According to this structures, a rectification smoothing circuit of each output channel can be suppressed in its peak value. Such power circuit can be smaller and more stable. Further, regulation characteristics are improved in an output voltage of each output channel.
In typicall power circuits, the input coil has a larger number of windings than the output coil corresponding to each output channel so that the output (secondary) side is a higher voltage than the input (primary) side. When the transformer has more output channels, the total number of output coil windings corresponding to output channels becomes larger. As mentioned in the above references, if the input coil is divided into two coils, output coils may not be accommodated within the width of the input coils. In this case, for example, a width of the output coil may be decreased by forming the output coils with double layers piled in a radial direction. However, the body of the transformer becomes larger, as the output coils become thicker. Such large body is not preferred in view of an accommodation space for mounting the transformer on a circuit board.
Further, since the relative position against the input coil is quite different in each output coil, a load variation in the input coil provides a different effect to output coils. In this case, the voltage variation should be individually compensated in each output channel. As a result, the power circuits become larger.